1. Field of the Invention
The invention is directed to a double-slit imaging system for measuring individual particle size, size distribution and particle velocity of particles suspended in a moving fluid. It is well suited for measuring particles of 3 .mu.m and larger, for example, in situ measurements in fossil fuel combustion systems.
2. Description of the Prior Art
The several existing methods for making single particle measurements belong to one of three categories, i.e., scattering methods, imaging methods and interferometric methods.
The prior art with respect to imaging methods is, so far as is known, the following.
Wang et al., "Applied Optics", Vol. 20, pgs. 1367-1373 (Apr. 15, 1981) is directed to particle size measurement using an optical variable-frequency-grid technique. In the procedure described therein as the image of a particle scans across a variable frequency grid, the transmitted light signal oscillates with varying visibility. The visibility goes through a null at a well-defined point when the particle diameter approximates the local grid spacing. The optical configuration employed by the present system is also an imaging method and is generally similar to that of the variable-frequency-grid (VFG) system, the variable frequency grid being replaced by a double-slit mask. However, the signal obtained by the use of the double-slit mask is entirely different from that obtained by a variable-frequency-grid, i.e., a double-pulsed output signal instead of the null signal of said VFG system.